It is known to provide a transmission line in the form of a so-called stripline, created by sandwiching a conductor between two conducting ground planes. Striplines are used in particular in the manufacture of compliant printed circuit boards (PCBs) in which the circuit required is patterned onto one face of a compliant substrate. A second unpatterned board of a similar compliant substrate is then bonded to the first using an electrically compatible bonding film which flows around the conductor pattern to create an homogeneous filling, i.e. one with no voids remaining.
To achieve more compact circuit board designs, high dielectric constant materials such as alumina, silicon, sapphire, gallium arsenide or gallium nitride may be used to construct the boards. However, such materials are generally rigid. If a stripline circuit pattern is formed on the surface of one rigid board and a second rigid board is bonded to the first in a similar manner to that for compliant boards, air filled voids are often created between the boards which degrade the electrical performance of the circuit. Bonding films and other adhesives do not in this case provide a solution since the dielectric constant of these materials is low compared to that of the material used to make the boards themselves.
There are a number of known arrangements for packaging high frequency devices. These may involve assembling each device into a ceramic package, for example, with tungsten wire feed-throughs to the device inside. Post-assembly lids are welded or otherwise adhesively attached to complete the package. In particular, it is known to mount high frequency devices on a circuit board and to interconnect them by stripline conductors patterned on the surface of the board. Each device may be individually packaged by bonding a cavitated lid to the board, under a vacuum, to cover the device. However, each lid must be bonded to the circuit board in such a way as to ensure a hermetic seal so that the vacuum under the lid is preserved. Typically, bonding is achieved using epoxies as they flow around the conductors to form a hermetic seal. However, where small volume vacuum sealing is needed, epoxies cannot be used as they out-gas into the package, over time, thus spoiling the vacuum. Similar problems can occur with soldered joints and glass frit joints.
Of necessity, high frequency stripline conductors are ‘thicker’ than conductors for use with lower frequency signals. This makes it difficult to achieve a good seal when bonding a lid over high frequency stripline conductors patterned onto the surface of a circuit board. Epoxies and glass frit seals tend not to flow-fill around the conductors, thus forming imperfect seals. To overcome this problem, schemes using conductor feed-throughs to the reverse side of a package are used by a number of manufacturers. This approach introduces further problems in sealing the feed-through. The approach also limits high performance to the lower microwave frequencies and below. Furthermore, where a number of separate lids are used to cover devices on a single circuit, then a second buried circuit layer becomes necessary to interconnect the feed-throughs to the reverse side of the board. This adds to fabrication complexity and degrades performance.